In an electrophotographic reproduction apparatus, in order to maintain good copy quality over a long period of time, the electrophotographic reproduction process is controlled according to environmental changes. More specifically, an optical sensor detects a reflectance of a bare surface of a photoreceptor and outputs a reference signal representing the reflectance. A specific control parameter of the electrophotographic reproduction process is changed in accordance with the reference signal. The control parameter is, for example, a lamp voltage, a voltage of a charger, a development bias voltage or a voltage of a discharger.
Copy quality deteriorates when the density of fog increases. The fog density is a lightness of a non-image area of a copy. An increase in the fog density is caused by various factors. The major factors are an increase in the residual potential of the photoreceptor and the deterioration of developer due to environmental changes or aged toner. A two-component series dry developer having toner and carrier is used in such an apparatus.
With a conventional process control, it is impossible to compensate for an increase in the fog density caused by the deterioration of toner due to heat history.
There may be a case where an electrophotographic reproduction apparatus is stored in a warehouse or transported under hot heat and where toner inside the apparatus is kept under a temperature of around 50.degree. for about two days. If the toner has such a heat history, the resin of the toner is softened and the charging characteristic of the toner deteriorates due to thermal stress. If the deteriorated toner is supplied, the fog density may increase even when the control parameter is properly set. Such an increase in the fog density can not be compensated by adjusting the lamp voltage and the development bias voltage.
A copy with an increased fog density would not appear to be right and the user judges that the copy has extremely low quality.
In addition, when the fog density is increased by the deteriorated toner, an erroneous reference signal may be delivered. If the control parameter is adjusted in accordance with such an erroneous reference signal, the copy quality would be degraded and an excessively great amount of toner is consumed for development. The excess toner results in uneconomical operation including flying of toner.
In recent years, an additive is used so that, even if toner has a heat history and deteriorates, it does not affect copy quality. However, the problem of high fog density has not yet been fully solved.